(1) Field of the Invention
The present invention relates to the field of nuclear medicine. More specifically, the present invention relates to the field of signal processing for scintillation detectors.
(2) Prior Art
Gamma camera detectors (e.g., of the Anger type) typically comprise a number of photomultiplier tubes (PMTs) arranged in a matrix in order to create the field of view for the detector. Each photomultiplier tube generates an output analog signal (channel signal) that is proportional to the intensity of light detected by each tube in response to a scintillation event within a crystal layer. As is well known, the scintillation event occurs as a result of an interaction between an emitted gamma ray and the crystal layer (which may be of NaI).
In the prior art, the signal output from each PMT are fed to circuitry that utilizes the signals to compute (1) a total energy of the interaction and (2) a spatial coordinate of the interaction. Circuitry for computing both of these values rely on an integration function of the analog signals from the PMT in order to derive the energy detected by each PMT. During periods when a particular PMT does not detect an event, its output signal is at a baseline voltage level (ideally zero). As more energy is detected by the PMT, its signal level rises accordingly.
Each PMT output signal may be called a "channel" signal and it is important that each channel signal have a predictable and reliable baseline voltage value because these channels signals will be independently integrated. Within the circuitry that is used to integrate each channel, there is typically also located circuitry for providing an independent gain for each channel signal. However, due to inherent characteristics and properties of the amplification circuitry for each channel, the channel signal may DC drift and have unwanted signal fluctuations as a result of many conditions, such as circuit temperature, variations in power supply current or voltages, noise associated with the amplification circuitry or the power supply, etc. This DC drift is undesirable since it will change the integration output of the channel signal and thereby decrease the accuracy of the positioning of the scintillation interaction.
What is needed is a circuit design that provides a constant and reliable baseline or offset DC voltage for each channel signal. Implementations to provide such constant baseline offset may be provided that utilize high performance and expensive amplifier circuitry with high tolerance levels. However, since a separate amplification stage is required for each PMT of the scintillation detector, such an implementation is not ideal due to cost factors; there may be as many as 55 or more PMTs associated with a single scintillation detector. Therefore, what is needed is a cost effective design for maintaining a channel signal at a constant and predictable baseline voltage level that prevents the channel signal from being clipped. The present invention offers such advantageous functionality.
Accordingly, it is an object of the present invention to provide more accurate signal processing within a gamma camera system. It is another object of the present invention to improve the accuracy of event energy and coordinate determination computations within a gamma camera. It is a further object of the present invention to provide channel circuitry for maintaining the DC baseline or offset voltage of a channel signal to a constant and predetermined value. It is another object of the present invention to perform the above task using relatively inexpensive amplification circuitry associated with each channel of a scintillation detector. It is an object of the present invention to utilize the above to provide an improved gamma camera system. These and other objects of the present invention not specifically mentioned above will become clear within discussions of the present invention herein.